A goal of roadway safety is to provide a forgiving roadway and adjacent roadside for errant motorists. Guardrails are employed along a roadside to accomplish multiple tasks. Upon vehicle impact, a guardrail must react as a decelerator and energy absorber to dissipate the kinetic energy of the vehicle. In addition, the guardrail acts as a mechanical guide to redirect the vehicle away from hazards during deceleration and to prevent the vehicle from leaving the road, being snagged by the guardrail system itself or from becoming airborne or rebounding excessively into traveled lanes of traffic.
For many years, various methods for the releasable mounting of guardrail system components have included the use of bolts that may fail (such as by stripping of the threads), break, or deform in a variety of relatively unreliable ways to accomplish the release of structural components. In some systems, a bolt is included that may sometimes shear and break, and at other similar times may deform as a washer passes through the post bolt slot of a guardrail panel to accomplish release. The washer sometimes pulls through the post bolt slot near the middle of the slot, and at other times pulls through the slot near an end of the slot, with quite significantly different release loads associated with each of these variations. In summary, the range of load types and magnitudes associated with each of these relatively unmanaged mechanisms may vary quite widely. With this, the actual release mode (the way that release is accomplished) has not been controlled or consistent, since it has not been unique or highly repeatable.
The reality has been that one of several release modes, or their various combinations, may actually cause release, depending upon a random combination of various extraneous variables that are also not well controlled during release. One example of release variables that are not well controlled in some guardrail systems involves bolt strengths. These are typically specified to be minimum values, such that actual bolt strengths may or may not be much higher. Details of bolt strength characteristics are typically only very roughly controlled. This means that there are extraneous combinations of various types of loads and geometric details that can and do occur. Moreover, each of these extraneous variables are further acted upon by installation details such as bolt position at the post bolt slot of a guardrail panel, and the tightness (torque) of a bolt when it is installed. These extraneous variables are all typical of weak post systems common in the United States.
In other guardrail systems, such as strong post systems, a block is provided between the guardrail and the post that pivots on the post in various ways, depending on actual guardrail forces and which side of the post the bolt is installed on, but generally providing various possible combinations of mechanical lever arms and respective fulcrums whereby the initially straight post bolt is bent and deformed, thus causing the solid head of the post bolt to pass through the post bolt slot, deforming the slot to accomplish the release of the guardrail panel from the post. Here again, the actual release mode is relatively unmanaged, and may vary widely. One very significant factor in some of these systems is whether the bolt head of the ⅝ inch diameter buttonhead bolt happens to pull through the guardrail post bolt slot near the center, or near an end of the slot. The difference in force magnitudes between these extremes may be as much as 60%. Moreover, this assumes that one is considering only the variation between extremes relative to a single ply of guardrail. The extremes are considerably wider when one compares the forces of a bolt pulling through the center of the slot of a single ply with the forces required to pull through the slot edge where there are two plies, such as at a splice, in which case the variation may be as much as 200 percent or more.
One common problem with these mounting methods has been the failure to achieve a reliable and repeatable release of the guardrail from the post even under relatively ordinary circumstances. The extremes of behavior in the prior art thus range between the adequate, to cases where the bolt head snags hard on one end of the post bolt slot, such that release may not occur at all. When effective release fails to occur, the guardrail may remain attached to a post far beyond optimum timing during a crash event.
The full importance of having a reliable and repeatable release mechanism has not been completely appreciated or understood in the highway safety industry. The result has been that errant vehicles struggle with the guardrail system in various ways during vehicle impacts rather than being smoothly redirected, simply due to inconsistent and relatively erratic release of components from each other during the vehicle impact event. The actual symptoms of unreliable release have been so commonly observed during vehicle crash tests that they have been categorized over the years by experienced crash testing engineers as vehicle vaulting, vehicle pocketing, or hard snagging of the vehicle wheel on various components such as posts. Significant suspension damage or occupant compartment deformation may also occur to the vehicle due to inadequate release. The vehicle itself may actually be destabilized by the action of excessive forces that pile up in the guardrail system, thus causing the vehicle to overturn or to exit the system at relatively high angles of roll, pitch, or yaw that at the very least may adversely affect the driver's efforts to control the vehicle.
The relative absence of reliable release is not a new problem. It has been a major problem that has plagued the worldwide highway safety industry since its inception over a hundred years ago. Extraneous forces related to inadequate release affect the successful functioning of the entire system at a basic level. Wide variations in release behavior have meant that important guardrail system forces remain largely unmanaged, making optimum and consistent system performance virtually impossible to achieve. Variations include unpredictable and undesirable force combinations and pile-ups when release fails to occur.
While the symptoms of these problems have long been recognized, the problem itself has largely remained undefined and not well understood, even to the point of being somewhat regarded as “stochastic”, intractable and possibly unsolvable. Within the United States as well as in Europe, many local variants of standard guardrail systems have cropped up, each representing the best local attempt to improve system capabilities.
Recently, there has been a vigorous effort to raise national performance standards that guardrails must satisfy. Increasingly stringent testing criteria have uncovered serious deficiencies in the capabilities of standard “W-beam” guardrail systems. Accordingly, recent efforts have focused on the development of new guardrail systems to accomplish safety goals more effectively.
In some cases, guardrail systems have actually been proposed for both weak and strong post systems that place the guardrail panel splices at the mid-span of the support posts in an attempt to reduce the variation of release forces at least somewhat by ensuring that the post bolt head must pass through no more than a single ply to accomplish release. In other cases, deeper blocks have been proposed in an effort to address the problems associated with the hard snagging of vehicle wheels on posts. None of these proposed approaches has gained wide acceptance, since they have represented only partial solutions to individual symptoms of the problem of inadequate release. Moreover, these solutions generally have had the effect of significantly increasing system complexity, cost, and installation time, without markedly increasing system capabilities.